1. Field of the Invention
The invention relates to an interferometer system that can be used to measure surface topography and surface movement with extreme accuracy and with sufficient rapidity to provide real-time feedback of changes in the topography or movement of the surface. In particular, the invention relates to an apparatus and method for using laser interferometry to obtain real-time feedback on: 1) the shape of a cornea as the cornea is being reshaped through a vision-correction process referred to as corneal photo-ablation, 2) the shape of precision manufactured components during manufacture, 3) non-contact measurement of vibration, and 4) movement in a rotating object.
2. Description of the Related Technology
In recent years laser interferometer devices have been developed for measuring a change in distance with extreme accuracy. Such devices generally direct a narrow laser beam onto a spot on a surface, detect the returned radiation, and use interferometry techniques to determine a change in distance along the optical axis of the laser beam. Such devices, however, typically only monitor the one spot, and do not measure the broader topography of the surface. As a result, things such as surface contours, rotational movement, and movement along any axis other than that of the laser beam are not detected.
Other devices, called topographers, have been developed for measuring surface contours. These are usually designed to map a stationary surface, so speed of operation is not critical. Topographers generally map the contours of a surface by determining the relative elevation of different points on that surface, and assuming a relatively constant slope between those two points. Elevation is defined by the distance from the point being measured to the measurement device, as measured along the axis of the returning signal. Since it is the change in elevation from one point to the next that defines the surface contour, it is generally only necessary to determine the relative change in elevation from point to point rather than the absolute elevation of any given point.
Topographers usually operate by directing a signal of some kind to a point on the surface to be mapped, detecting the signal bounced back from that point, and repeating the process for various other points. Points at different elevations will return different signals. By comparing the difference in signals returned from different points, the difference in elevation between those points can be determined. With enough data points, the contour of the entire surface can be mapped.
To measure the contours of a surface with microscopic accuracy, a topographer should be able to determine the relative elevation of a large number of individual points on that surface with extreme precision. Laser interferometers are known for their ability to make precise measurements of a change in elevation of a single point. A simple Michelson interferometer can be used to illustrate the basic technique as shown in FIG. 1a. The output from a laser 105 is divided into two beams by beam splitter 103. The first of the resulting beams is reflected back to beam splitter 103 by mirror 101, which is fixed and thus produces a fixed optical path length. The second beam is reflected back to beam splitter 103 by mirror 102, which can be moved along the optical axis as shown by the arrow. This movement changes the optical path length of the second beam by twice the amount of movement. Portions of these two reflected beams are combined at beam splitter 105 and this combined beam is sent to a detector 104. As mirror 102 is moved along the optical axis, the intensity at the detector varies as shown in FIG. 1b, due to the manner in which the phases of the two return beams combine. A mirror movement of one half the laser wavelength causes one full oscillation cycle in the measured intensity. Mirror movement is determined by counting the number of intensity oscillations, or xe2x80x98fringesxe2x80x99. Hence, physical movements can be accurately detected with this method within a fraction of one-half wavelength by examining the phase position within a fringe cycle. Since the wavelength of laser radiation is typically on the order of a micron, changes of a fraction of a micron can be accurately measured in this manner. However, this technique is effective only if the vertical step size (the change in elevation between successive measurements) is less than one-half wavelength. Since the fringe counter is based on phase differences, multiples of one-half wavelength cannot be detected and only the residual fraction of one-half wavelength will be measured.
Using a Michelson interferometer is an accurate method of measuring a change in elevation of a given point, as that elevation changes with time. Unfortunately, it requires the mirror (or other optically smooth surface) to be perpendicular to the beam, or the reflected light will miss the sensor. However, the same basic principles can be used for detecting laser light that is scattered from an optically rough surface. Optically rough surfaces scatter light by returning it in many different directions at once. However, by using a small-diameter laser beam and a small detector, only the light scattered along a narrow, predictable return path will be detected, thereby simulating parallel reflected light. Although the returned light might be much weaker than with a reflected signal, this is offset by the benefits of detecting light from spots that are not perpendicular to the laser beam.
The fixed mirror of the Michelson interferometer can be replaced with a flat but optically-rough reference surface, while the moveable mirror can be replaced with an optically-rough non-flat target surface to be measured. Instead of moving the target surface along the optical axis, the small-diameter laser beam can be scanned across the target surface, causing changes in elevation on the surface to produce the same fringe effects previously noted. By controlling the scan pattern so that each fringe measurement corresponds to a known location on the target, the surface contours of that target can be accurately mapped. However, this technique has the same limitations noted above. If the vertical step size, or change in elevation between sequentially measured spots, is greater than one-half wavelength, the fringe counter will only detect the residual fraction of half a wavelength. Similar confusion results if an individual spot being measured contains variations in elevation larger than one half wavelength.
This is one of the major drawbacks of laser-based topographers. Most lasers that are suitable for this application operate with wavelengths in the micron and submicron range. Thus any change in elevation between two sequentially-measured points must be much less than a micron or it won""t be accurately measured. For most applications, this requires that the two adjacent points be very close together, to avoid the cumulative effects of moving up or down a sloping surface. However, this closeness increases the number of points that must be measured for a given surface area. Using a standard rectangular grid of measurement points, as the spacing between measurement points decreases by a given proportion, the amount of measuring and processing will increase as the square of that proportion. Thus the processing becomes unwieldy and slow, and the time it takes to do a single surface scan increases accordingly. For mapping the surface of a small stationary object, this may not be objectionable. But mapping the surface of an object with a changing surface requires completing sequential scans in real time so the changes in that surface with time can be determined. In these cases, the short wavelength of the laser beam can be a detriment because it requires more closely spaced measurement points, which increases the time to complete each scan. The operator must sacrifice speed for accuracy, or vice-versa.
To solve these problems, the laser radiation needs to have a wavelength which is at least twice as large as the maximum surface roughness (elevation variation) expected to be encountered between reasonably-spaced measurements points. Unfortunately, lasers which are suitable for this application all have comparatively short wavelengths. Long wavelength lasers exist, but it is not possible to focus a long wavelength laser to a small enough spot for most applications. Many potential applications for laser interferometer topography cannot use conventional laser topographers because the expected surface variation of the target is too great.
To overcome all the problems noted above, a device and process is needed that uses an interferometer that can measure surface contours over a variety of roughness ranges that provides this measurement data quickly enough to rapidly make successive scans, and that measures enough points to minimize the likelihood that undetected irregularities between measured points will unacceptably degrade the results. To use the techniques of laser interferometry in this application, a light source is needed that provides a monochromatic beam with the wavelength stability of laser light, but with a wavelength longer than that permitted by the requirement to focus the laser to a small spot. This wavelength should preferably also be adjustable to accommodate different roughness ranges without requiring a major system redesign.
The present invention addresses the aforementioned problems in the prior art through a unique combination of laser interferometry, optical superheterodyning, and synthetic wavelength generation. This process is referred to as xe2x80x9csuperheterodyne synthetic wavelength interferometryxe2x80x9d. A system implementing this process can include a scattered-light interferometer, a superheterodyne optical system for generating and using synthetic wavelength beams, an interferometer control system that controls the interferometer for surface topography measurements, a feedback control system, and a target control system for manipulating or modifying the target surface. Most of these components may be similar in all applications. However, the target control system may vary greatly, depending on the application. For instance, a medical system for reshaping the cornea of a human patient is much different that a system for manufacturing precision metal components.
The laser source may be any wavelength-stabilized laser, but preferred lasers may include such types as distributed feedback diode lasers (DBF), distributed Brag reflector diode lasers (DBR), extended cavity diode lasers, or diode pumped solid state lasers. These are all well-known laser types that are commercially available. Eye-safe lasers, such as those with a wavelength near 1550 nm are preferred for safety reasons. To make the topographer""s laser radiation suitable for particular applications, optical superheterodyne techniques may be used to synthetically increase the laser""s effective wavelength until it is long enough to detect the anticipated elevation changes and surface irregularities.
The invention may use four optical frequencies: f1, f2, f3 (which may be a frequency-shifted f1), and f4 (which may be a frequency-shifted f2). Unlike conventional devices, which send f1+f2 to the target surface and f3+f4 to the reference surface, the invention may send f1+f2 to both the target and reference surfaces, and mix their returned signals with a local oscillator signal f3+f4.
The interferometer may measure optically-rough target surfaces and use an optically-rough reference surface, thus eliminating the need for either the target surface or the reference surface to be precisely perpendicular to the laser beams.
The interferometer may use counter-rotating, wedges to generate the scan pattern.
The interferometer may operate with two synthetic wavelength beams having, different synthetic wavelengths, with the difference between the two synthetic wavelengths being much smaller than either synthetic wavelength. The first synthetic wavelength may be formed by using a first polarizing beam splitter to combine a p-polarized portion of a first laser beam with an s-polarized portion of a second laser beam. The linear polarized components may be converted to left and right circular polarized components using a first quarter wave plate. After passing through a non-polarizing beam splitter, focus control, and scanning mechanism, this mixture of optical signals may be directed to both the target surface and the reference surface with a second polarizing beam splitter. The portions of these signals which are scattered back from the two surfaces can be directed back along the same path to the non-polarizing beam splitter.
The second synthetic wavelength may be formed by using a third polarizing beam splitter to combine an s-polarized frequency-shifted portion of the first laser beam with a p-polarized frequency-shifted portion of the second laser beam. The frequency shifts can be accomplished with first and second acousto-optic modulators operating at different frequencies. The resulting linear polarized components may be converted to left and right circular polarized components using a second quarter wave plate. After passing through the non-polarizing beam splitter, the left and right circularized components may be mixed to form the second synthetic wavelength.
The second synthetic wavelength and the returned components of the first synthetic wavelength may be combined in the non-polarizing beam splitter and passed through a third polarizing beam splitter where they may be separately directed to a target surface signal sensor and a reference surface signal sensor. The outputs of the two sensors may then be analyzed to detect fringe patterns.
One embodiment of the invention can be used in a system to ablate corneal tissue, by using the interferometer system to map the topography of the surface of an ablated cornea, determining the difference between the surface contour and an expected contour, changing the ablation parameters based on this difference, and continuing the ablation based on the changed parameters.
Another embodiment of the invention can be used in a system to manufacture precision machine parts by using the interferometer system to map the topography of the surface of a part during machining, determining the difference between the surface contour and an expected contour, changing the machining parameters based on this difference, and continuing the machining operation based on the changed parameters.
Another embodiment of the invention can be used in a system to measure surface vibrations by using the interferometer system to measure the topography of multiple points on the test object, repeating the measurements at short time intervals, and using the difference in elevation between successive measurements for each point to analyze the vibration characteristics of that point. The invention can also be used to measure the transverse motion of a rotating point (i.e., a point on a rotating wheel) by scanning the laser beam in a circular pattern, and synchronizing the rotational speed of the scan with the rotational speed of the target so that the laser beam continues to impinge on a single point of the rotating target.
It is an object of the invention to provide a topographer to accurately measure the contours of a surface.
It is a further object of the invention to measure elevation changes in the surface to an accuracy of less than two microns.
It is a further object of the invention to provide data on these measurements quickly enough and often enough to interactively control a process of modifying the surface.
It is a further object of the invention to measure enough points on the surface to detect correctable irregularities on that surface.
It is a further object of the invention to measure the shape of a surface being changed by radiation, such as a cornea during an ablation process, and use the measured shape to interactively control the ablation process.
It is a further object of the invention to measure the shape of a mechanical part being changed by mechanical means, such as a machined part during a machining operation, and use the measured shape to interactively control the mechanical means.
It is a further object of the invention to measure the vibration characteristics of multiple points on a vibrating object, without physically contacting the object with measurement devices.
It is a further object of the invention to measure the motion of at least one point on a rotating surface in a direction parallel to the axis.